The present invention concerns a tensioning screw for orthodontic teeth straightening devices.
Tensioning screws of this type are known from the present applicant's U.S. Pat. No. 4,483,674. They are employed to adjust the situation and position of teeth or groups of teeth along the curve of the jaw. They include a nut and a threaded shaft that extends through it, tensioned between two points within the device. The effective length of the shaft is shortened during treatment to adjust the situation and position of the teeth.
Such screws have proved effective in practice for this purpose. Still, the shaft will sometimes automatically turn backward, or even slowly out of the nut, due for example to the wearer's chewing motions. Attempts have been made to prevent this by using self-locking threads, but they are complicated to manufacture and make the screws more expensive. It also becomes almost impossible to screw the screw backwards when necessary, and, when one has been screwed in too tight, it must usually be discarded.